A study of the spatial variation of electric field in highly resistive metal films by scanning tunneling potentiometry

Abstract

Electronic transport in highly resistive (but metallic) thin platinum films (≈ 10 nm) deposited by electronbeam evaporation has been studied by scanning tunneling microscopy and scanning tunneling potentiometry (STP). The films have an average grain size of ≈ 10 nm. On this scale transport through the film is very inhomogeneous. Scattering from grain boundaries (GBs) results in large variations in the local potential resulting in fields as high as 10 4 -10 5 V/cm located near the GB. The reflection coefficient R g of electrons at a GB has values between 0.5-0.7 as determined from the STP data. This can be compared to an average R g ≈ 0.9 obtained from an analysis of the bulk resistivity data taken over the temperature range 4.2 K < T < 300 K.Charge transport in thin metal films is not uniform on the nanometer scale and is accompanied by strong inhomogeneities in the field and current density near localized scatterers such as grain boundaries (GBs) and defects. Conventional theories of transport ignore these spatial variations and consider the averaged field. In this average picture all details of the charge transport in the conductor are parameterized by a single parameter, namely the mean free path of the charge carrier. This describes the correct picture in a bulk system but as the sample dimension decreases it becomes necessary to measure the relevant quantities on a nanometer scale and the deviations of the local fields and potentials from the average values become significant. The significance of the influence of localized scatterers was first recognized and investigated theoretically by Landauer [1]. He pointed out that the microscopic dipole field that results from the asymmetry of the electron scattering at a point defect is the ultimate source of the residual resistivity. Work of Landauer and subsequent developments [2,3] showed the equivalence of the residual resistivity dipole approach and the more conventional Boltzmann transport approach. Understanding the exact microscopic nature of the scattering process is important in order to gain insight into fundamental questions like charge transport at the nanometer scale and is also of practical interest in phenomena like electromigration. While bulk transport properties which give a measure of the average behavior have been investigated for a long time, the actual electronic processes taking place at a nanometer level (at the scale of electronic mean free path) remained essentially unanswered mainly because of the lack of an experimental tool to study the properties on this scale. Development of scanning probe microscopies (SPMs) [4,5] and in particular scanning tunneling potentiometry (STP) [6-8] has enabled us to experimentally observe the scattering-induced field variations at a nanometer scale. More recent theoretical studies have established the condition under which scanning tunneling microscopy (STM) can measure the change in local electrochemical potential [9]. It is only recently with the advent of STP that this issue is being reinvestigated and a...

A Novel Study of the Growth and Resistivity of Nanocrystalline Pt Films Obtained from Pt(acac)2 in the Presence of Oxygen or Water Vapor

“…F was experimentally determined to be equal to 0.5±0.7 for platinum, [37] and we used 0.6. With r bulk = 10.7 lX cm, we obtained perfect agreement with experimental data for crystallite dimensions greater than 10 nm, irrespective of all the experimental conditions used, but below this value the resistivity rapidly increases without obeying the theoretical trend, therefore we applied a corrective factor, k/(2 D), to values under 10 nm.…”

Development of Scanning Tunneling Potentiometry for Semiconducting Samples

“…To obtaining STM data in the ac mode, the modulation voltage is added to the sample bias voltage, and in order to pick up the resulting ac component in the tunneling current, we inserted an absolute amplifier before the STM feedback, instead of a lock-in amplifier, which is often used to extract an ac component, because of its fast response and simplicity. 9) Since the frequency of the modulation is higher than the STM feedback response, the averaged absolute current, which is proportional to the differential tunneling conductance, is utilized for the tip height control. The lower part of Fig.…”

“…As one of the SPM techniques, scanning tunneling potentiometry (STP) uniquely provides nanoscale mappings of the potential landscape induced by current flow in thin films or surface layers, and has been used on various materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] It was first invented by Muralt and Pohl in 1986, 1) and later Kirtley et al demonstrated its high performance by visualizing potential drops at grain boundaries in a AuPd film, 3) which clearly indicated the finite electrical resistance there. A recent STP work on graphene also elucidated the potential drops at grain boundaries.…”

Electrical and thermal conductivities of polycrystalline platinum nanowires

“…The reflection coefficients for various metallic NFs have a large span. Taking Pt NFs as an example, the reflection coefficient ranges from 0.15 to about 0.8 [7][8][9].…”